1. Field of the Invention
The present invention relates to a stereo camera apparatus and a vehicle-mountable monitoring apparatus using the stereo camera apparatus, and more particularly, to a compact stereo camera apparatus of reduced size that can enhance measurable distance range and angle of view.
2. Description of the Background Art
More and more vehicles such as automobiles have been equipped with safety devices to enhance vehicle safety. For example, a forward-watching device, such as a television camera or a laser-equipped radar unit, may be mounted on a vehicle to detect vehicles and/or obstructions or objects in front of the vehicle. The information gained by such detection can be used to determine a degree of danger or hazard level such as collision risk and activate a warning signal to a driver, activate an automatic braking system to decrease vehicle speed, or automatically increase or decrease vehicle speed (or cruising speed) to maintain a predetermined vehicle-to-vehicle distance with the vehicle ahead. Research into and development of such technologies is being actively pursued, under the concept of an advanced safety vehicle (ASV), for example.
Such technologies, which can detect objects in front of the vehicle using images captured by a television camera mounted on the vehicle (e.g., an automobile), are known. For example, JP-H05-265547-A discloses one vehicle-mountable monitoring apparatus that mounts four stereo cameras: two stereo cameras for long-range (telephoto) detection using charge coupled devices (CODs), and two more stereo cameras for short range (wide-angle) detection also using CODs. Each of the stereo cameras captures images of one or more objects, and the captured images are processed to compute distance to the objects from the vehicle as object distance distribution information. The computed object distance distribution information can be further used to compute three-dimensional positions of both objects and road environment, such as road width, for example.
In such a configuration, the long-range stereo cameras have an object detectable range such as from 20 m to 100 m ahead of the vehicle on which they are mounted, and the short-range stereo cameras have an object detectable range such as from 5 m to 30 m, for example.
The stereo cameras for long-range can detect far objects such as 100 m ahead of the vehicle, but cannot detect too-close objects at distances such as 20 m or less ahead of the vehicle. On the other hand, the stereo cameras for short range can detect close objects ahead of the vehicle by using a wide-angle lens, which have a broader field of view and thus can detect objects in a broader lateral range to the sides of the vehicle. Further, the two stereo cameras for short range are disposed closer to each other and can thus detect objects close to the vehicle.
To maintain a broad field of view for short distance and long distance, the two stereo cameras for long range and the two stereo cameras for short range may be used selectively depending on changes in vehicle environment and conditions. For example, when a vehicle is traveling on a highway at high speed, the stereo cameras for long-range detection are activated for monitoring the road in front of the vehicle, whereas when the vehicle is traveling on a local road at low speed, the stereo cameras for short range are activated for monitoring the road in front of the vehicle.
Further, JP-H11-39596-A discloses another vehicle-mountable monitoring apparatus, in which two stereo cameras for long range and two stereo cameras for short range are concurrently activated to capture images of objects, instead of selectively using the stereo cameras for long- and short-range depending on changes in vehicle environment and road conditions. Such vehicle-mountable monitoring apparatus prepares a distance-information included image for long range and a distance-information included image for short range. The distance-information included image for long range is segmented into a plurality of sections (e.g., a grid), in which one whole captured image is divided into multiple sections of a given size. Similarly, the distance-information included image for short range is also segmented into a plurality of sections (e.g., a grid) of a given size for dividing one captured image into the plurality of sections, in which one whole captured image is divided into multiple sections having a given size. Then, for each of the sections, image data on or above a road face is extracted as data for three-dimensional objects and distance to the object is computed.
As above described, the vehicle-mountable monitoring apparatus mounted on vehicle for watching a broader field of view for short distance and long distance may have a relatively greater size. This factor is important because, generally, stereo cameras are disposed at a front top position in a vehicle, such as the position of the rear-view mirror of an automobile. The position of rear-view mirror is close to a head position of the driver's seat and the passenger seat in view of the function of the rear-view mirror, which is used to provide a view to the rear of the vehicle. A big stereo camera mounted at such rear-view mirror position may oppress the driver and/or the passenger and can pose a safety risk for both driver and passenger.
JP-2004-258266-A discloses a stereo camera having two lenses, mirrors disposed in a light path for reflecting light coming from the two lenses, and one image capturing element. When light enters and passes one lens, the light is reflected by one mirror and guided to the single joint image capturing element. When light enters and passes the other lens, the light is reflected by the other mirror and also guided to the single joint image capturing element. Such a configuration can reduce the number of image capturing elements and reduce the size of stereo camera. The smaller the size of the stereo camera, the smaller the moment force of stereo camera, in which the stereo camera can attain a higher rigidity when vibration or acceleration forces act on the stereo camera.
As described above, JP-H05-265547-A and JP-H11-39596-A disclose vehicle-mountable monitoring apparatuses using two stereo cameras for long range and two stereo cameras for short range, meaning four stereo cameras in total, to ensure a broader field of view at both long and short ranges. However, such a configuration requires a greater number of parts for the optical systems. For example, the number of lenses and image capturing elements may be twice or more that of ordinary stereo cameras, by which manufacturing cost of the vehicle-mountable monitoring apparatuses may increase.
Also as described above, JP-2004-258266-A discloses a stereo camera using two lenses and one image capturing element, in which a light path coming from the lenses to the image capturing element is tilted for some angle, by which a size of stereo camera is reduced, and manufacturing cost is also reduced. In such a stereo camera, a baseline length set between two lenses (i.e., an interval of two lenses) may be set shorter so that one image capturing element can be used for two lenses. However, due to the principle of triangulation, a shorter baseline length may hinder a distance measurement for far objects.